In general, images used for official documents, such as corroborating photographs, need to have originality sufficiently ensured. Since digital images can be altered relatively easily without any strange appearance, however, the digital images have insufficient admissibility of evidence in some cases. Therefore, technologies for ensuring the originality of digital images and for detecting unauthorized alteration of digital images are demanded. As a method for achieving these objects, electronic signatures which use a hash function have been conventionally examined, but electronic signatures are not intended to detect a position where unauthorized alteration was made.
As a method for detecting a position where unauthorized alteration was made, electronic watermarking has attracted attention conventionally. Electronic watermarking is classified according to characteristics into two types, “robust” and “fragile”. Robust electronic watermarking is robust against attacks and is mainly used for copyright protection. To detect a position where unauthorized alteration was made, fragile electronic watermarking is used. Fragile electronic watermarking is very sensitive to image processing. During a process to detect unauthorized alteration, an electronic watermark which has been changed somehow can be identified to determine a position where an unauthorized alteration was made. Several references are listed below.                Japanese Unexamined Patent Application Publication No. 2002-44429        Japanese Unexamined Patent Application Publication No. 2002-201703        Hideaki Tamori, Naoshi Aoki, and Tsuyoshi Yamamoto, “Electronic Watermarking Technique Capable of Detecting Unauthorized Alteration, Using Number Theoretic Transform,” Technical Research Report of the Institute of Electronics, Information and Communication Engineers, IE2001-33, pp. 105-110, July, 2001        Hideaki Tamori, Naoshi Aoki, and Tsuyoshi Yamamoto, “Unauthorized-Alteration-Position Detection and Unauthorized-Alteration Correction in Still Images, Using Fragile Electronic Watermarking with Number Theoretic Transform,” Technical Research Report of the Institute of Electronics, Information and Communication Engineers, IE2002-45, pp. 19-24, July, 2002        H. Tamori, N. Aoki, and T. Yamamoto, “A Fragile Digital Watermarking Technique by Number Theoretic Transform,” IEICE Trans. Fundamentals, August 2002        